Continuous flow water heater



April 1, 1958 A. L. MILLER 2,828,723

' CONTINUOUS FLOW WATER HEATER Filed July 29, 1954 INVENTOR. I I

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rates and 3 Claims. (Cl. 122-264) The present invention relatesgenerally to water heaters, and more particularly to such devices of thecontinuous flow type.

Continuous flow Water heaters are those in which water is heated onlyduring the supply thereof as opposed to storage type heaters in whichthe water contained in the storage tank is maintained at the desiredsupply temperature, and the Water is recirculated to bring thetemperature up to the desired hot water temperature whenever a portionof the contents of the storage tank is withdrawn.

One of the principal disadvantages encountered in the use of continuousflow heaters, particularly those which are gas fueled, is the fact thatone end of the water tube is, due to the incoming cold water, constantlyat a temperature considerably below the dew point of the combustionproducts surrounding the outside of the tube, while the other end of thetube is usually somewhat above the temperature of the desired hot wateroutput due to the fact that the flow rate is fairly slow and of laminarrather than turbulent flow, thus reducing the efiiciency of heattransfer between the tube and the water inside the same. Thus, in short,one end of the water tube is too cold and the other end too hot.

Both the low temperature at the incoming end of the boiler tube and thehigh temperature at the opposite end cause operational difiiculties inaddition to a loss of heating efiiciency. At the cold end of the tube,condensate is formed on the outer surface along a substantial length ofthe tube due to the fact that the products of combustion are heavilysaturated with water vapor, and upon being cooled to the temperature ofthe inflowing water, condense out most of their moisture. This moisturedrips down on the burners into the lining of the fire box, and ingeneral creates serious corrosion problems.

In order to reduce as much as possible the effects of exteriorcondensation as above described, it has been the practice in someconventional continuous flow heaters, to bypass a major portion of thetotal flow of water through the heater and heat only a relatively smallamount of such total flow to a high enough temperature so that theheated water, when mixed with the bypassed water produces the desiredoutput temperature. Heating the water to such relatively hightemperatures reduces the length of the cold portion of the heater'tubeswhich are below the dew point temperature of the flue gases, and thussomewhat reduces the effects of condensation. Such expedient onlysubstitutes another difficulty, however, that of the deposit of mineralson the interior of the heater tubes. As is well known, the deposit ofminerals in so-called hard waters, increases as the temperature of thewater increases. Thus, the above-described expedient of heating a smallportion of the total supply to a relatively high temperature mayinitially be fairly effective insofar as condensation is concerned, butin a relatively short time the interior of the tubes, due to the veryhigh temperatures of the water atet ice therein, become clogged withmineral deposits and the entire system breaks down. Furthermore, even avery slight deposit of minerals acts as-an insulator and greatly reducesthe heat transfer efficiency.

Another practical disadvantage of continuous flow water heaters ofthetype heretofore available, has been the low efficiency of heattransfer between the hot products of combustion and the water tubes. Inan effort to increase this efficiency, water tubes have been provided,carrying an external helical fin, thus to increase the heat transfersurface on the exterior of the tube. Because it is impractical toconcentrate the externally applied heat on asingle flow tube, it hasbeen the usual practice to provide a number of parallel water tubes,finned as above described, through all of which the water flows inparallel paths, the tubes being arranged side-by-side in the fire boxabove the burners.

While the provision of external fins in the manner above describedgreatly increases the external area of the water tube, and thus improvesthe elficiency insofar as any particular tube is concerned, these finsinterfere with the close spacing of the tubes and thus-permit aconsiderable amount of hot gas to pass between the tubes without comingin intimate contact with either the tubes themselves or the finsthereon. While it would be theoretically possible to arrange the finnedtubes in sideby-side relationship with the fins on one interlacedbetween the fins on the adjacent tube, such mounting presents manypractical difiiculties, and in any event creates too great a restrictionto the hot gases at the point where tube fins are interlaced or meshedwith the fins of an adjacent tube.

Bearing in mind the above-mentioned difficulties encountered incontinuous flow water heaters of the class described, it is a majorobject of the present invention to materially increase the efiiciency ofa continuous flow water heater.

It is another object of the invention to provide an arrangement in acontinuous flow water heater for preventing wide temperaturedifferentials in the heating tubes, thus to eliminate condensation ofmoisture on the exterior thereof and deposit of minerals on the interiorthereof.

It is still another object of the invention to provide a continous flowwater heater in which the rate of flow through the heater tubes isseveral times greater than that dictated by the rate of hot waterdelivered whereby to decrease mineral deposits and increase the thermaletficiency at the heat exchange surfaces of the heater tubes.

A still further object of the invention is to provide for turbulent flowthrough the heater tubes of a continuous flow water heater whereby toincrease the thermal efficiency.

Yet a further object of the invention is to provide a water tubemounting-fire box assembly wherein optimum contact with the hot gases isachieved while presenting a minimum constriction to the flow of gasesaround the tubes.

A still further object of the invention is to provide a water tubemounting assembly in a continuous flow heater wherein it is possiblewith a minimum of disassembly to clean the interior of the tubes.

The foregoing and additional objects and advantages of the inventionwill be apparent from the following detailed description of a presentlypreferred embodiment thereof, consideration being given likewise to theaccompanying drawings in which:

Figure 1 is a partially cut away perspective view of a continuous flowwater heater embodying the present invention;

Figure 2 is an enlarged fragmentary elevational section taken on alongitudinal plane through the water tube assembly and illustrating theflow path thereto; and

Figure 3 is an elevational transverse section taken on the line 3-3 inFigure 2.

Referring to Figure 1 in the drawings, it will be seen that the heaterembodying the present invention is enclosed in an outer, generallyrectangular sheet metal housing 10, having gas fired burners 11 near thebottom of an internal fire box 15, and horizontal water tubes 12, 13,and 14, extending longitudinally across the fire box 15 near the top.The burners 11 and the water tubes 12, 13, and 14, are completelyenclosed by the fire box 15 which is lined with refractory tile andinsulated from the outer housing 10 by a layer of rock wool 16 or thelike. An interior hood 17 of sheet metal serves to collect the fluegases after they have passed in contact with the water tubes 12, 13, and14, and direct such gases to the flue 18 at the top of the housing 10.Fuel gas is supplied by a fuel line 21 and conventional gas controlelements are provided in the form .of a manual and automatic pilotcontrol valve connected to the gas supply 21, a pressure regulator 22, asolenoid control 23 for the main burner, and a conventional safety pilot24. Since the just-destcribed elemtns do not form an important part ofthe present invention, no further detailed description thereof is deemednecessary herein.

The water tubes 12, 13, and 14, are arranged in three transverse banks,one above the other, as can be seen in Figure 3, the lowermost bankbeing designated by the reference character 12, the intermediate bankbeing designated at 13, and the top bank being designated at 14. It willbe noted that bottom bank of tubes 12, and the top bank 14 are providedwith heat transfer fins 25 which are coined from the parent metal of thetube by a rolling operation, while the intermediate tubes 13 have noexternal fins. The relative arrangement of the lower bank of tubes 12,the intermediate bank 13, and the top bank 14 is such that theperipheries of the fins on the upper and lower banks 12 and 14 are closeto, but not quite in tangential contact with the intermediate tubes 13.

I have found that, contrary to what might be expected, a much greaterheating efiiciency for a given rate of gas consumption can be achievedby the use of an intermediate bank of unfinned tubes staggered withrespect to the tubes in the other banks as shown in Figure 3 than can beachieved with an equivalent total number of tubes, all of which havefins. This appears to be due to the fact that the intermediate unfinnedtubes 13 act as baffles, diverting and causing the hot flue gases toflow in curvilinear paths as shown by the arrows in Figure 3 whereby topass the gases in considerably more intimate contact with the totalsurface of the fins on the finned tubes than would be the case if nointermediate tubes 13 were present. Separate bafiles, not containingwater to be heated could be employed, but the heat consumed in heatingthe baffles would be of course be wasted.

If it is attempted to arrange eleven finned tubes in the general patternindicated in Figure 3 whereby an intermediate bank of finned tubes wouldserve to a certain extent as bafiies, it is found that the increase inefiiciency is not achieved due to the geometry of the finned tubes andthe necessary increase in spacing therebetween. On the other hand, if itis attempted to use all finned tubes, and to intermesh the fins on theintermediate banks with those of the banks above and below, theconstriction to the flow of flue gases which is produced by theintermeshing of the fins, is such as to greatly reduce the efficiency.

The tubes 12, 13, and 14, are mounted between headers 28 and 29 as bybrazing indicated at .30, which headers form ends for the fire box 15and are hollow to provide water manifold chambers 31 and 32 whichrespectively divide the fiow and recombine it as water passes throughthe heater.

The cold water input to the heater is at the left end of the devicethrough an external input manifold 33, having an internally threadedpipe connection 34 to receive the input pipe (not shown). The inputmanifold 33 forms a separate chamber from the manifold chamber 31 in theheader 28.

The input manifold 33 is bolted to the lefthand header 28 as by bolts35, thus forming a closure for the upstream manifold chamber 31. Asimilar closure is provided for the chamber 32 in the form of a plate 39bolted to the header 29. With both the input manifold 33 and the plate39 removed, free access is given for cleaning the tubes 12, 13, and 14,as by reaming, or otherwise.

Communication from the interior of the input manifold 33 to the manifoldchamber 31 in the header is provided through a plurality of jet nozzles37 which are secured in threaded openings in the inner wall 38 of theinput manifold. The arrangement of the jet nozzles 37 in the wall 38 issuch that each one of them aligns axially with one of the tubes 12 and13 in the bottom and intermediate banks of tubes. No nozzles areprovided in alignment with the upper bank of tubes 14. The internal boreof each nozzle 37 is relatively small whereby to produce a relativelyhigh velocity jet of cold water introduced into each of the tubes 12 and13. The kinetic energy in such water jet acts in the manner of aninjector pump to draw water from the manifold chamber 31 into therespective tubes 12 and 13 whereby the water flowing from left to rightthrough the tubes 12 and 13 constitutes a mixture of incoming cold waterfrom the jet'nozzles 37 and the contents of the manifolding chamber 31.Such mixture is, due to the action of the jet nozzles 37, highlyturbulent thus promoting a much greater efficiency of heat transferbetween the water and the interior walls of the tubes 12 and 13 thanwould be the case with smooth or laminar flow.

The water issuing from the tubes 13 and 14 into the downstream manifoldchamber 32 passes upwardly where the flow divides, a portion of theheated water passing out through the output connection 40 and conduit41, and the other portion of the flow passing back to the upstreammanifold chamber 31 through the top bank of tubes 14.

The back pressure in the hot Water supply line (output) 41 is such as tocause a substantial portion of the how to recirculate from right to leftthrough the top bank of the tubes 14. To correctly proportion the flowdivision just described, a restricting orifice member may be inserted inthe output connection 40 if needed. The portion of the flow passing fromright to left through the top bank of tubes 14 is further heated by theflue gases, and thus the temperature of the water in the upstreammanifold chamber 31 at the left header 28 is the highest in the entiresystem. Such high temperature water mixed with the cold input waterissuing from the jets 37 so raises the temperature of the water Withinthe tubes 12 and 13, that the portion thereof which is below the dewpoint temperature of the line gases is substantially eliminated. Thus,the condensation of moisture on the exterior of the tubes 12 and 13 isvirtually nil. Since the temperature of the water passing through theupper tubes 14 is even greater than that at the lefthand end of thetubes 12 and 13, there is no condensation on the top bank of tubes 14.

By an appropriate adjustment of the back pressure in the hot watersupply conduit 41, the proportion of the flow which passes in a counterdirection through the upper tubes 14 can be adjusted. It has been foundthat with the arrangement of 11 tubes illustrated in Figure 3, anoptimum overall efiiciency is achieved when the flow rate is such as tocause approximately 25% of the total heat transfer to occur in the topbank of tubes 14. This percentage can be determined by measurement offlue gas temperatures at various points in the tube banks.

By way of example, but not by way of limitation, an

adjustment as just described when used in connection with aheater suchas shown in Figure 1 adapted to heat 4.8 gallons of water per minute ata temperature rise of 50 F. was found to have a water temperature riseas between the right and left ends respectively of the top bank of tubes14 of only 2. This relatively small temperature rise in the counter flowtubes 14 indicates the high flow rate through the individual tubes ascompared to the overall effective flow rate through the entire system.Putting it another way, the sum in the flow rates of the individualtubes is much greater than the flow rate at which heated water isdelivered atthe output 41. If the water flowed from left to right in alltubes as in most conventional heaters, the sum of the flow rates wouldmerely equal the delivery rate and the fiow rate through any tube wouldbe very much smaller than in the present case, and the efiiciencycorrespondingly reduced. The above-mentioned high rate of flow in theheater described herein has the advantages already mentioned of greatlyreducing the deposition of minerals on the interior walls of the tubes,and also producing and augmenting the turbulence within the tubes thusto increase the heat transfer efliciency.

While the heater construction shown and described herein is fullycapable of achieving the objects and providing the advantageshereinbefore stated, it is capable of some modification withoutdeparture from the spirit of the invention. For this reason I do notmean to be limited to the forms shown and described, but rather to thescope of the appended claims.

I claim:

1. In a continuous flow water heater: an input chamber having means toreceive water to be heated; an output chamber having means to deliverheated water from the heater; a plurality of substantially parallelwater tubes substantially horizontally disposed and arranged insuecessively vertically displaced banks; each of said tubes having openends communicating directly respectively with said input chamber andsaid output chamber; said means to receive water to be heated comprisinga plurality of jet nozzles, each positioned opposite and directedrespectively toward a corresponding one of the plurality of tubes in atleast the lowermost of said banks; at least the uppermost of said banksof tubes having no obstruction in said input chamber opposite the openends of said tubes; the open ends of the tubes in said output chamberbeing unobstructed; said jet nozzles being arranged to eifect highvelocity circulation of water through at least said plurality of tubesin said lowermost bank to said output chamber; means to pass a hot fluidupwardly around said banks of tubes to heat the same; whereby theapparatus is adapted to induct heated water into tubes in at least thelowermost bank wherein the same is mixed with injected unheated waterand the mixture is passed at a high velocity to the outlet chamber, apart of the water is recirculated through tubes in the uppermost bank tothe input chamber, and at least a portion of the highly heated waterentering the input chamber from the upper bank of tubes is drawndownwardly and said induction takes place.

2. The apparatus of claim 1 in which said plurality of substantiallyparallel and horizontal water tubes are of equal diameter; the tubes inalternate banks having external circumferential heat transfer finsthereon and the tubes in any intermediate bank being unfinned; the tubesbeing spaced apart, in a horizontal direction, a distance substantiallyequal to the diameter of said tubes; the finned tubes being in verticalalignment with one another and spaced apart in the vertical direction adistance substantially equal to the diameter of said tubes; saidunfinned tubes being in vertical alignment with the spaces between saidfinned tubes; whereby the tubes in one bank act as bafiles for said hotfluid passing transversely of said tubes to divert the said hot fluidinto intimate heat transfer contact with the tubes in the nextsucceeding bank.

3. The apparatus of claim 1 in which said jet nozzles are mounted in aremovable plate defining at least a portion of one wall of said inputchamber.

References Cited in the file of this patent UNITED STATES PATENTS834,444 Bausher et a1. Oct. 30, 1906 1,027,327 English May 21, 1912FOREIGN PATENTS 510 Switzerland Oct. 4, 1904 28,953 Switzerland Apr. 16,1903 116,209 Great Britain June 6. 1918 282,717 Great Britain May 3,1928 603,646 Great Britain June 21, 1948 675,924 Great Britain July 16,1952

